Current traditional energy technologies rely on fossil fuels. Their most significant limitations are the depletion of limited fossil fuel reservoirs, thus, making this a non-sustainable technology, and the net generation of CO2 and other greenhouse gases, thereby affecting the global climate in a fundamental and uncontrollable manner. Hydrogen gas is a renewable energy source that does not evolve the “greenhouse gas” CO2 in combustion, liberates large amounts of energy per unit weight in combustion, and is easily converted to electricity by fuel cells.
However, current sources of H2 often rely on fossil fuels as input material. The use of H2 as a large scale fuel therefore depends, in part, on developing new H2 sources. One path of particular interest is biological H2 production from sunlight, enabled by genetically engineered photosynthetic microbes that express hydrogenases—enzymes that catalyze the reversible reduction of protons into H2.
Biological H2 production has several advantages over H2 production by photoelectrochemical or thermochemical processes. Biological H2 production by photosynthetic microorganisms, for example, requires the use of a simple solar reactor such as a transparent closed box, with low energy requirements. An ideal process to produce H2 more economically would be to use water as an input, and photosynthetic processes to generate the energy needed for reduction of protons to form H2.
Naturally occurring photosynthetic organisms are unable to meet this need. Cyanobacteria such Synechocystis species have both photosystem I (PS I) and photosystem II (PS II) and can oxidize water to generate photoreductants. However, most H2-evolving hydrogenases are extremely sensitive to O2, which is an inherent byproduct of cyanobacterial photosynthesis. Therefore, to establish a successful cyanobacterium-bacterium hybrid system using H2O as the electron donor, one critical requirement is to use a hydrogenase that is not only tolerant to O2 but also catalytically active in the presence of O2.
O2-tolerant hydrogenases with useful rates of H2 production have not been found in nature, and so there is considerable interest in the genetic engineering of such a protein. The present invention addresses these problems.